When an injectable preparation is to be administered to a patient by means of a hypodermic injection, it is often important that the injection is given at a correct location below the surface of the patient's skin. This means that it is necessary to administer the injection in such a way that the injectable preparation is expelled at a correct depth under the surface of the skin. If the injected preparation is administered at an incorrect depth, i.e. too superficial or too deep, this may mean that the injection will have an inferior effect, or have unexpected side effects. In both cases, this may lead to serious consequences for the patient.
This is not an important problem when injections are administered by a trained physician or nurse, such as in a hospital. However, in recent years, a number of devices have been developed, by means of which a patient can administer injections to himself, such as in the ambulatory treatment of diabetes with insulin. These devices usually comprise a holder for a standard cartridge of an injectable preparation, and a dosing device, which can be set to measure and administer a predetermined dose of the preparation. An injection needle or cannula is attached to one end of the cartridge to establish a connection with the interior of said cartridge, and by means of the dosing device, the set dose of the preparation is expelled from the cartridge through the needle. These injection devices are very practical for the patient, but as the patient usually does not have any medical training, there is a risk that he will insert the needle to an incorrect depth, so that the injection is not given at the correct location under the skin.
Another drawback of the known injection devices for self-administering is that the attached injection needle is fully exposed. Many persons find it very unpleasant to administer injections to themselves in this way. There seems to be a psychological block in many persons against penetrating one's own skin with a fully exposed needle. Because of this, there have for certain applications been developed auto-injectors, where a concealed needle is made to penetrate the patient's skin when a spring arrangement is released. Such arrangements, however, are complicated and expensive, and are not suitable for self-injection devices where a number of injections are to be administered from one cartridge.